Authors: Alexandra Guebert, BASc ^∗, Michael Roumeliotis, PhD^∗,†,‡, Che Hsuan David Wu, MD^†, Karen Long, CMD ^‡, Natalie Logie, MD^†,‡, Tannis Graham, RTP^‡, Alison Gourley, BSc^‡, Peter Craighead, MD^†,‡, Michael Sia, MD^†,‡, Sarah Quirk, PhD^{∗,†,‡,1}

^∗ Department of Physics & Astronomy, University of Calgary, Calgary, AB, Canada
^† Department of Oncology, University of Calgary, Calgary, AB, Canada
^‡ Tom Baker Cancer Centre, Calgary, AB, Canada

Abstract:
To inform clinical practice for women receiving post-mastectomy radiotherapy (PMRT), this study demonstrates the dosimetric impact of removing daily bolus on skin and subcutaneous tissue. Two planning strategies were used: clinical ?eld-based (n = 30) and volume-based planning (n = 10). The clinical ?eld- based plans were created with bolus and recalculated without bolus for comparison. The volume-based plans were created with bolus to ensure a minimum target coverage of the chest wall PTV and recalculated without bolus. In each scenario, the dose to super?cial structures, including skin (3 mm and 5 mm) and subcutaneous tissue (a 2 mm layer, 3 mm deep from surface) were reported. Additionally, the difference in the clinically evaluated dosimetry to skin and subcutaneous tissue in volume-based plans were recalculated using Acuros (AXB) and compared to the Anisotropic Analytical Algorithm (AAA) algorithm. For all treatment planning strategies, chest wall coverage (V90%) was maintained. As expected, super?cial structures demonstrate signi?cant loss in coverage. The largest difference observed in the most super?cial 3 mm where V90% coverage is reduced from a mean (± standard deviation) of 95.1% (± 2.8) to 18.9% (± 5.6) for clinical ?eld-based treatments with and without bolus, respectively. For volume-based planning, the subcutaneous tissue maintains a V90% of 90.5% (± 7.0) compared to the clinical ?eld-based planning coverage of 84.4% (± 8.0). In all skin and subcutaneous tissue, the AAA algorithm underestimates the volume of the 90% isodose. Removing bolus results in minimal dosimetric differences in the chest wall and signi?cantly lower skin dose while dose to the subcutaneous tissue is maintained. Unless the skin has disease involvement, the most super?cial 3 mm is not considered part of the target volume. The continued use of the AAA algorithm is supported for the PMRT setting.

As of January 1, 2022, ARRT requires CE Credits for Directed Journal Readings to be based on the word count for each article. So, the number of CE Credits for each DJR article will vary for ARRT. For this article, the ARRT CE Credit will be 0.75 Credit. The MDCB CE Credits will remain at 2.5 Credits. 

ARRT CQR Credit Distribution
Radiation Therapy 2017:
Procedures
     Prescription and Dose Calculation = 0.75 credits
Radiation Therapy 2022:
Procedures
     Prescription and Dose Calculation = 0.75 credits